This invention relates to the production of starch hydrolysates such as glucose syrups, maltodextrins and dextrose.
It has long been known to convert starch with dilute acid and/or enzymes so as to make syrup, and the annual production of starch syrup in the United States is over 3 billion pounds per year. Typical processes of the known type are described in Langlois, Canadian Pat. No. 618,164 and Barton et al Canadian Pat. No. 753,227. Because of its greater commercial importance and availability, corn starch is used to the greatest extent for conversion into syrup and starch syrups are for the most part corn syrups.
Starch syrups which are manufactured by a straight acid conversion process correspond closely in properties when converted to the same degree. For example, corn syrup produced by straight acid conversion, having a dextrose equivalent (DE) of 42%, will be found to contain about 22% dextrose, 20% maltose, 20% higher sugars and 38% dextrins. All 42 DE corn syrups prepared by acid hydrolysis have essentially the same composition. Likewise, starch syrup manufactured by the combination acid and enzyme conversion process will correspond fairly closely in properties, although this type of process affords a wider range in the properties obtained, depending on the ratio of the conversion obtained by the acid step to that obtained by the enzyme converting step.
Generally starch hydrolysis products, such as syrups can be divided into four components, namely: dextrose, a monosaccharide; maltose, a disaccharide; higher sugars, which include the trisaccharides and tetrasaccharides; and dextrins, which include all polymers higher than tetra-saccharides. Dextrose represents the ultimate degree of conversion or hydrolysis of starch, whereas dextrins represent the smallest degree of conversion or hydrolysis. Maltose and the higher sugars represent intermediate degrees of conversion or hydrolysis.
Both the chemical and physical properties of any particular starch syrup will depend primarily upon its particular content of these four main components. Since dextrose is sweeter than maltose, and maltose is sweeter than the higher sugars and dextrins, which are nearly tasteless, it follows that the higher dextrose-maltose content a syrup has, the sweeter it will be. The maximum sweetness is attained in a syrup having maximum dextrose content. On the other hand, dextrins are composed of relatively large molecules and therefore dextrins contribute viscosity or body to a syrup, whereas dextrose and maltose molecules do not. A syrup which has a high dextrin content will have a greater viscosity and will have more body to it than one which has a low dextrin content. In addition to sweetening power and viscosity or "body", which have been mentioned, other important properties of syrups include reducing power, humectant character, fermentability, osmotic pressure and freezing point, depending upon the use which is to be made of the syrup.
The technology of enzymes in the production of starch hydrolysates has been developed to a high degree in recent years. Thus, one widely used procedure is to use a combination acid and multiple enzyme conversion process while other procedures involved the use of enzymes both for liquefaction and saccharification.
Starches are, of course polymers of anhydroglucose units which are linked through alpha-glucosidic bonds. Most starches contain two types of polymers, namely amylose and amylopectin. The former is a linear polymer in which the monomeric units are linked essentially through alpha-1, 4-glucosidic bonds. The presence of hydroxyl groups in the amylose chain imparts hydrophilic properties to the amylose polymer which leads to an affinity for moisture and resulting solubility in hot water. However, since the amylose molecules are linear and contain hydroxyl groups, they have a tendency to be attracted to each other and to align themselves by the association, as, for example, by hydrogen bonding, through the hydroxyl groups on neighbouring molecules. When this occurs, the affinity of the amylose polymers for water is reduced and, if the molecules are in solution, they will tend to come out of solution forming precipitates at dilute concentrations. These precipitates consist of three dimensional polymeric networks held together by spot hydrogen bonding particularly at higher concentrations where the motion of the amylose polymers and their ability to orient is more restricted. This phenomenon of molecular association through hydrogen bonding as manifested by crystallization from aqueous dispersions is commonly referred to as "retrogradation". Thus, for example, the tendency of corn starch dispersions to become opaque on cooling and to form gels is a result of retrogradation of the amylose molecules which are present in the corn starch.
Amylopectin, the other polymer which is present in the starch molecule, contains a predominance of 1,4 linked anhydroglucose units, but in addition, at about every 15th anhydroglucose unit there is a branch point extending from the 6th position of the anhydroglucose unit to the 1 position of the branching chain. Amylopectin is a larger polymer than amylose, and is believed to attain molecular weights in the millions. The highly branched structure of amylopectin keeps its molecules from approaching each other closely enough to permit the extensive hydrogen bonding necessary for retrogradation to occur. As a result, aqueous sols of amylopectin, or starches wherein amylopectin is the primary or sole component, are characterized by good clarity and stability.
One of the problems with enzymes in hydrolyzing starch is that most enzymes are unable to act on the retrogradation products. Moreover, there are many different enzymes capable of hydrolyzing the glucosidic linkages in the starch molecules, but most of these enzymes are unable to hydrolyse all of the 1,4 or 1,6 bonds. Either an equilibrium condition occurs at a certain degree of conversion or the enzyme is incapable of hydrolyzing certain specific linkages at various loci in the amylose or amylopectin chain. Another disadvantage of certain enzyme systems is that they contain trans-glycosidase activity which results in the synthesis of appreciable amounts of di- or trisaccharides from dextrose or maltose, thereby preventing complete hydrolysis to dextrose. It is also believed that these problems are compounded by the fact that the starch molecules have a curled or spiral configuration which interferes with ready access by the enzymes to the reactive points where hydrolysis would normally occur.
It is an object of the present invention to provide an improved process for producing starch hydrolysates of the above type.